Summary Atmospheric carbon dioxide concentration ([CO2]) is increasing, which increases leaf‐scale photosynthesis and intrinsic water‐use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO2] increase and thus climate change. However, ecosystem CO2 responses are complex or confounded by concurrent changes in multiple agents of global change and evidence for a [CO2]‐driven terrestrial carbon sink can appear contradictory. Here we synthesize theory and broad, multidisciplinary evidence for the effects of increasing [CO2] (iCO2) on the global terrestrial carbon sink. Evidence suggests a substantial increase in global photosynthesis since pre‐industrial times. Established theory, supported by experiments, indicates that iCO2 is likely responsible for about half of the increase. Global carbon budgeting, atmospheric data, and forest inventories indicate a historical carbon sink, and these apparent iCO2 responses are high in comparison to experiments and predictions from theory. Plant mortality and soil carbon iCO2 responses are highly uncertain. In conclusion, a range of evidence supports a positive terrestrial carbon sink in response to iCO2, albeit with uncertain magnitude and strong suggestion of a role for additional agents of global change.

The aim of our study was to identify interactions between the decomposition of aboveground litter and rhizosphere activity. The experimental approach combined the placement of labelled litter (delta13C=-37.9 per thousand ) with forest girdling in a 35-year-old Norway spruce stand, resulting in four different treatment combinations: GL (girdled, litter), GNL (girdled, no litter), NGL (not girdled, litter), and NGNL (not girdled, no litter). Monthly sampling of soil CO2 efflux and delta13C of soil respired CO2 between May and October 2002 allowed the partitioning of the flux into that derived from the labelled litter, and that derived from native soil organic matter and roots. The effect of forest girdling on soil CO2 efflux was detectable from June (girdling took place in April), and resulted in GNL fluxes to be about 50% of NGNL fluxes by late August. The presence of litter resulted in significantly increased fluxes for the first 2 months of the experiment, with significantly greater litter derived fluxes from non-girdled plots and a significant interaction between girdling and litter treatments over the same period. For NGL collars, the additional efflux was found to originate only in part from litter decomposition, but also from the decay of native soil organic matter. In GL collars, this priming effect was not significant, indicating an active role of the rhizosphere in soil priming. The results therefore indicate mutual positive feedbacks between litter decomposition and rhizosphere activity. Soil biological analysis (microbial and fungal biomass) of the organic layers indicated greatest activity below NGL collars, and we suppose that this increase indicates the mechanism of mutual positive feedback between rhizosphere activity and litter decomposition. However, elimination of fresh C input from both above- and belowground (GNL) also resulted in greater fungal abundance than for the NGNL treatment, indicating likely changes in fungal community structure (i.e. a shift from symbiotic to saprotrophic species abundance).

SummaryFor accurate interpretation of oxygen isotopes in tree rings (d 18 O), it is necessary to disentangle the mechanisms underlying the variations in the tree's internal water cycle and to understand the transfer of source versus leaf water d 18 O to phloem sugars and stem wood. We studied the seasonal transfer of oxygen isotopes from precipitation and soil water through the xylem, needles and phloem to the tree rings of Larix decidua at two alpine sites in the L€ otschental (Switzerland

SummaryElevated CO 2 increases intrinsic water use efficiency (WUE i ) of forests, but the magnitude of this effect and its interaction with climate is still poorly understood.We combined tree ring analysis with isotope measurements at three Free Air CO 2 Enrichment (FACE, POP-EUROFACE, in Italy; Duke FACE in North Carolina and ORNL in Tennessee, USA) sites, to cover the entire life of the trees. We used d 13 C to assess carbon isotope discrimination and changes in water-use efficiency, while direct CO 2 effects on stomatal conductance were explored using d 18 O as a proxy. Across all the sites, elevated CO 2 increased 13 C-derived water-use efficiency on average by 73% for Liquidambar styraciflua, 77% for Pinus taeda and 75% for Populus sp., but through different ecophysiological mechanisms. Our findings provide a robust means of predicting water-use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and speciesspecific relationships that can help modelling elevated CO 2 and climate impacts on forest productivity, carbon and water balances.

Summary• Woody species in Mediterranean ecosystems form intra-annual density fluctuations (IADFs) in tree rings in response to changes in environmental conditions, especially water availability.• Dendrochronology, quantitative wood anatomy and high-resolution isotopic analysis (using a laser ablation technique) were used to characterize IADFs in Arbutus unedo shrubs grown on two sites with different water availability on the island of Elba (Italy).• Our findings show that IADF characterization can provide information about the relationship between environmental factors and tree growth at the seasonal level. At the more xeric site, IADFs mainly located in the early and middle parts of the annual ring, showed a decrease in vessel size and an increase in d 13 C as a result of drought deficit. Opposite trends were found at the more mesic site, with IADFs located at the end of the ring and associated with a lower d 13 C. Moreover, at the first site, IADFs are induced by drought deficit, while at the second site IADFs are linked with the regrowth in the last part of the growing season triggered by favourable wet conditions.• This combined approach is a promising way for dating problematic wood samples and interpreting the phenomena that trigger the formation of IADFs in the Mediterranean environment.

Rising atmospheric [CO2 ], ca , is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO2 ], ci , a constant drawdown in CO2 (ca  - ci ), and a constant ci /ca . These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying ca . The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to ca . To assess leaf gas-exchange regulation strategies, we analyzed patterns in ci inferred from studies reporting C stable isotope ratios (δ(13) C) or photosynthetic discrimination (∆) in woody angiosperms and gymnosperms that grew across a range of ca spanning at least 100 ppm. Our results suggest that much of the ca -induced changes in ci /ca occurred across ca spanning 200 to 400 ppm. These patterns imply that ca  - ci will eventually approach a constant level at high ca because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant ci . Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low ca , when additional water loss is small for each unit of C gain, and increasingly water-conservative at high ca , when photosystems are saturated and water loss is large for each unit C gain.

Intra-annual density fluctuations (IADFs) in tree rings are generally considered structural anomalies caused by deviations from the “normal course” of xylogenesis during the growing season. This definition is based on the bias that, under “normal conditions”, cambial activity stops once a year. Each tree ring can thus be dated to one calendar year, which is one of the principles of dendrochronology. The formation of IADFs can be triggered directly by environmental changes, especially in precipitation and temperature, that affect cambial activity and cell differentiation. It can also be the result of limited photosynthesis, due to defoliation induced by biotic or abiotic constraints.Often indicated with alternative terms, IADFs were first described in the 1930s, and recently reported for many trees and shrubs from different ecosystems throughout the world, particularly for Mediterranean species. Different types of IADFs have been detected; their formation and structural properties depend on many factors including tree genotype, age, size, rooting depth, habitat, soil, climate, photosynthetic activity, and allocation strategies. Whether IADFs affect the adaptive capability of plants remains, however, unclear.We provide an overview of the main anatomical features of IADFs and their occurrence in tree rings from various environments and climatic regimes. We propose a simplified way of classifying them and discuss the hypotheses about their functional role and the factors triggering their formation. To understand the ecological role of IADFs better, we recommend a multidisciplinary approach, involving wood anatomy, dendroecology, and stable isotopes, which has already been applied for Mediterranean species. We conclude by considering that IADFs appear to be the “rule” rather than “anomalies” in some ecosystems where they help plants cope with fluctuating environmental conditions. Moreover, their anatomical structure represents a valuable proxy of past climatic conditions at a sub-seasonal resolution and may be relevant to adapt hydraulic functioning of living trees to changing climatic conditions.

[1] Isotopic discrimination measurements in tree rings are becoming increasingly important estimators of past environmental change. Potential biases inherent to these parameters, including age trend and level offset are, however, not well understood. We here perform measurements on a new millennium-long data set of decadally resolved d 18 O and d 13 C discrimination from 25 high-elevation pine trees in the Spanish Pyrenees to investigate whether such low-frequency biases exist and how they alter the long-term behavior of derived time series. Alignment of the tree ring data by biological age reveals age trends over the first one to four centuries after germination. On average, isotope values change by −0.089‰ d 18 O and +0.064‰ d 13 C per decade over the first 100 years of tree growth. This trend persists into the forth century after germination for d 18 O but diminishes to ∼0‰ over the 100-390 year period for d 13 C. We also find level offsets up to 7‰ d 18 O and 3‰ d 13 C between single trees. Analysis of the shape of age trends reveals negative exponential functions as reasonable choices for detrending of d 18 O and (inverted) d 13 C time series. The detrended isotope chronologies share low-frequency variance with traditional ring width and density measurements beyond statistical chance, suggesting that the various parameters reflect some of the same environmental forcing. Our results show that d 18 O and d 13 C from these Spanish pines need to be detrended to assess long-term environmental changes. To evaluate the general applicability of this conclusion, production of (1) well-replicated, (2) nonpooled, and (3) composite chronologies from other species and regions will be required. Increases in measurement speed and technology will make these tests feasible in the coming years.